The present invention relates to a novel positive photosensitive composition sensitive to a light ray in a wavelength region of from 650 to 1300 nm. More particularly, it relates to a positive photosensitive composition suitable for direct plate making by means of a semiconductor laser or a YAG laser, a positive photosensitive lithographic printing plate employing the composition and a method for making a positive photosensitive lithographic printing plate.
Along with the progress in the image treating technology by computers, an attention has been drawn to a photosensitive or heat sensitive direct plate making system wherein a resist image is formed directly from digital image information by a laser beam or a thermal head without using a silver salt masking film. Especially, it has been strongly desired to realize a high resolution laser photosensitive direct plate making system employing a high power semiconductor laser or YAG laser, from the viewpoint of downsizing, the environmental light during the plate making operation and plate making costs.
On the other hand, as image-forming methods wherein laser photosensitivity or heat sensitivity is utilized, there have heretofore been known a method of forming a color image by means of a sublimable transfer dye and a method of preparing a lithographic printing plate. Known as the latter is, for example., a method of preparing a lithographic printing plate by means of the curing reaction of a diazo compound (e.g. JP-A-52-151024, JP-B-2-51732, JP-A-50-15603, JP-B-3-34051, JP-B-61-21831, JP-B-60-12939 and U.S. Pat. No. 3,664,737), or a method of preparing a lithographic printing plate by means of the decomposition reaction of nitrocellulose (e.g. JP-A-50-102403 and JP-A-50-102401).
In recent years, a technique in which a chemical amplification type photoresist is combined with a long wavelength light ray absorbing dye, has been proposed. For example, JP-A-6-43633 discloses a photosensitive material wherein a certain specific squarilium dye is combined with a photo-acid-generator and a binder.
Further, as a technique of this type, a technique for preparing a lithographic printing plate by exposing a photosensitive layer containing an infrared ray absorbing dye latent Bronsted acid, a resol resin and a novolak resin, in an image pattern by e.g. a semiconductor laser has been proposed (JP-A-7-20629). Further, the same technique wherein a s-triazine compound is used instead of the above latent Bronsted acid, has also been proposed (JP-A-7-271029).
However, these conventional techniques were not necessarily adequate in their performance from a practical viewpoint. As a more serious problem, in the case of such a chemical amplification type photosensitive plate, it was usually essential to have a heat treatment step after exposure, and due to variation of heat treatment conditions or the like, the stability in the quality of the image thereby obtainable was not necessarily adequate, and a technique containing no such a step has been desired. In the above-mentioned JP-A-7-20629 and JP-A-7-271029, a method for obtaining a positive image without requiring the above-mentioned post heat treatment, is proposed, but no specific Examples are given, and no specific method or no fact of obtaining such a positive image is disclosed. Further, in such a technique, the photosensitive material is sensitive also to ultraviolet light, and it is necessary to carry out the operation under yellow light containing no ultraviolet light, such being problematic from the viewpoint of the operation efficiency.
Further, in U.S. Pat. No. 5,491,046, a plate-making method particularly an exposure method, using such a composition is disclosed, but no Example is given for a positive image.
Further, JP-A-60-175046 discloses a radiation sensitive composition comprising an alkali-soluble phenol resin and a radiation sensitive onium salt, which is photo-dissolvable. It is disclosed that in the composition, photo-decomposable decomposition of the onium salt induces the resin to regain the solubility, to satisfy the basic requirement for a photo-dissolvable system, and that the onium salt can be sensitized by an electromagnetic spectrum of a wide range ranging from ultraviolet light to visible light or even to infrared light.
Such an image is formed essentially by a difference in the solubility in a developer as between an exposed portion and a non-exposed portion. For such a difference to be caused, it is common that one of the components in the composition undergoes a chemical change, and to induce such a chemical change, an additive such as a photo-acid-generator, a radical initiator, a crosslinking agent or a sensitizer, is frequently required, whereby there has been a problem that a system will be complicated.
The present invention has been made in view of the above-described various problems.
Namely, it is an object of the present invention to provide a positive photosensitive composition and a positive photosensitive lithographic printing plate, which are simple in their construction, which are suitable for direct recording by e.g. a semiconductor laser or a YAG laser and which have high sensitivity and excellent storage stability.
Another object of the present invention is to provide a novel positive photosensitive material and a positive photosensitive lithographic printing plate, which are highly sensitive to an infrared ray and which require no post exposure heat treatment.
A further object of the present invention is to provide a photosensitive material and a positive photosensitive lithographic printing plate, which do not require an operation under yellow light and whereby the operation can be carried out under usual white light containing ultraviolet light.
A still further object of the present invention is to provide a positive photosensitive lithographic printing plate which is excellent in a burning property as a lithographic printing plate.
Still another object of the present invention is to provide a plate-making method, whereby a positive photosensitive lithographic printing plate can be exposed at high sensitivity.
Such objects of the present invention can be accomplished by the following constructions of the present invention:
A positive photosensitive composition showing a difference in solubility in an alkali developer as between an exposed portion and a non-exposed portion, which comprises, as components inducing the difference in solubility,
(a) a photo-thermal conversion material, and
(b) a high molecular compound, of which the solubility in an alkali developer is changeable mainly by a change other than a chemical change.
A positive photosensitive composition comprising a photo-thermal conversion material and an alkali-soluble resin and having a characteristic represented by B less than A where A is the solubility, in an alkali developer, at an exposed portion of the composition, and B is the alkali solubility after heating of the exposed portion.
A positive photosensitive lithographic printing plate having such a positive photosensitive composition formed on a support.
A method for making a positive photosensitive lithographic printing plate, which comprises a step of scanning and exposing such a positive photosensitive lithographic printing plate by means of a light ray belonging to a wavelength region of from 650 to 1100 nm and having a light intensity sufficient to let the high molecular compound form an image.
Now, the present invention will be described in detail with reference to the preferred embodiments.
Heretofore, as a positive photosensitive composition, a system has been known which comprises an alkali-soluble resin and an o-quinone diazide group-containing compound as a photosensitivity-imparting component. It is believed that with this system, upon irradiation of ultraviolet light which can be absorbed by the o-quinone diazide group-containing compound, the diazo moiety will decompose to finally form carboxylic acid, whereby the alkali-solubility of the resin increases, so that only the exposed portion will dissolve in an alkali developer to form an image. Further, in the composition disclosed in the above-mentioned JP-A-60-175046, the photo-decomposable decomposition of the onium salt contributes to the solubility of the resin. Namely, in these systems, a component in a photosensitive composition undergoes a chemical change.
Surprisingly, the present invention provides a photosensitive composition capable of forming a positive image with a very simple system of a photo-thermal conversion material and an alkali soluble resin where no chemical change is expected.
The reason as to why the photosensitive composition of the present invention provides such an excellent effect is not clearly understood. However, it is considered that the light energy absorbed by the photo-thermal conversion material is converted to heat, and the alkali-soluble resin at the portion subjected to the heat undergoes a change other than a chemical change, such as a change in conformation, whereby the alkali solubility at that portion increases, so that an image can be formed by an alkali developer.
Such an effect is attributable mainly to a change other than a chemical change. This is assumed, for example, from a reversible phenomenon such that when a photosensitive composition of the present invention once irradiated, is heated around 50xc2x0 C. for 24 hours, the alkali solubility of the exposed portion once increased immediately after the exposure, often returns to a state close to the state prior to the exposure. Thus, the present invention provides a positive photosensitive composition comprising a photo-thermal conversion material and an alkali-soluble resin, which has a characteristic represented by B less than A, where A is the solubility, in the alkali developer, at an exposed portion of the composition, and B is the alkali solubility after heating of the exposed portion. Further, the relation between the glass transition temperature (or the softening temperature) of the photosensitive composition itself and the likelihood of the reversible phenomenon, was examined, whereby it was found that the lower the temperature, the more likely the phenomenon. This also supports the above-described mechanism.
Accordingly, it should be understood that the essential constituting components of the positive photosensitive composition of the present invention are a photo-thermal conversion material of component (a) and a high molecular compound of component (b) only, and a material which increases the alkali solubility of an alkali-soluble resin by an action of active radiation, such as the above-mentioned o-quinone diazide group-containing compound, or a material such as a combination of a compound (a photo-acid-generator) which forms an acid by active radiation, with a compound, of which the solubility in a developer increases by an action of the acid, is not substantially required. Further, the positive photosensitive composition of the present invention is used exclusively for forming a positive image, and a material which becomes insoluble in a developer by an action of active radiation, such as a diazo resin, a crosslinking agent and a combination of an ethylenic monomer with a polymerization initiator, which are used as components of a negative photosensitive composition, and a sensitizer for activating them, are also not substantially required. Thus, the composition of the present invention is clearly distinguished also from a photosensitive composition which is useful as both positive and negative photosensitive compositions. Further, the composition of the present invention does not contain a compound susceptible to a-photochemical sensitizing effect by the photo-thermal conversion material and is clearly distinguished from the composition disclosed in JP-A-60-175046.
The positive photosensitive composition of the present invention may contain a solubility-suppressing agent (dissolution inhibitor) which is capable of lowering the alkali solubility of the photosensitive layer prior to exposure, as described hereinafter.
Now, the photo-thermal conversion material (hereinafter referred to as a light-absorbing dye) as the first component used for the positive photosensitive composition of the present invention, will be described. This material is not particularly limited so long as it is a compound capable of converting absorbed light to heat. However, it is preferably a light-absorbing dye (a) having an absorption band covering a part or whole of a wavelength region of from 650 to 1300 nm. The light-absorbing dye to be used in the present invention is a compound which effectively absorbs light in a wavelength region of from 650 to 1300 nm, while it does not substantially absorb, or absorbs but is not substantially sensitive to, light in an ultraviolet region, and which will not modify the photosensitive composition by a weak ultraviolet ray which may be contained in white light. Specific examples of such a light-absorbing dye will be presented in Table 1. 
These dyes can be prepared by conventional methods.
Among these, a cyanine dye, a polymethine dye, a squarilium dye, a croconium dye, a pyrylium dye and a thiopyrylium dye are preferred. Further, a cyanine dye, a polymethine dye, a pyrylium dye and a thiopyrylium dye are more preferred.
Among these, particularly preferred is a cyanine dye of the following formula (I) or a polymethine dye of the formula (II) in a wavelength region of from 650 to 900 nm, and a pyrylium dye or a thiopyrylium dye of the following formula (III) in a wavelength region of from 800 to 1300 nm: 
wherein each of R1 and R2 is a C1-8 alkyl group which may have a substituent, wherein the substituent is a phenyl group, a phenoxy group, an alkoxy group, a sulfonic acid group, or a carboxyl group; Q1 is a heptamethine group which may have a substituent, wherein the substituent is a C1-8 alkyl group, a halogen atom or an amino group, or the heptamethine group may contain a cyclohexene ring or a cyclopentene ring having a substituent, formed by mutual bonding of substituents on two methine carbon atoms of the heptamethine group, wherein the substituent is a C1-6 alkyl group or a halogen atom; each of m1 and m2 is 0 or 1; each of Z1 and Z2 is a group of atoms required for forming a nitrogen-containing heterocyclic ring; and Xxe2x88x92 is a counter anion. 
wherein each of R3 to R6 is a C1-8 alkyl group; each of Z4 and Z5 is an aryl group which may have a substituent, wherein the aryl group is a phenyl group, a naphthyl group, a furyl group or a thienyl group, and the substituent is a C1-4 alkyl group, a C1-8 dialkylamino group, a C1-8 alkoxy group and a halogen atom; Q2 is a trimethine group or a pentamethine group; and Xxe2x88x92 is a counter anion. 
wherein each of Y1 and Y2 is an oxygen atom or a sulfur atom, each of R7, R8, R15 and R16 is a phenyl group or a naphthyl group which may have a substituent, wherein the substituent is a C1-8 alkyl group or a C1-8 alkoxy group; each of l1 and l2 which are independent of each other, is 0 or 1; each of R9 to R14 is a hydrogen atom or a C1-8 alkyl group, or R9 and R10, R11 and R12, or R13 and R14, are bonded to each other to form a linking group of the formula: 
wherein each of R17 to R19 is a hydrogen atom or a C1-6 alkyl group, and n is 0 or 1; Z3 is a halogen atom or a hydrogen atom; and Xxe2x88x92 is a counter anion.
The counter anion Xxe2x88x92 in each of the above formulas (I), (II) and (III) may, for example, be an inorganic acid anion such as Clxe2x88x92, Brxe2x88x92, Ixe2x88x92, ClO4xe2x88x92, BF4xe2x88x92 or PF6xe2x88x92, or an organic acid anion such as a benzenesulfonic acid, p-toluenesulfonic acid, naphthalene-1-sulfonic acid or acetic acid.
The proportion of such a light-absorbing dye in the positive photosensitive composition of the present invention is preferably from 0.1 to 30 wt %, more preferably from 1 to 20 wt %.
Now, the high molecular compound (hereinafter referred to as a polymer or a resin) (b), of which the solubility in an alkali developer is changeable mainly by a change other than a chemical change, as the second component used for the positive photosensitive composition of the present invention, will be described. As such a polymer, alkali-soluble resins such as a novolak resin, a resol resin, a polyvinyl phenol resin and a copolymer of an acrylic acid derivative, may, for example, be mentioned. Among them, a novolak resin or a polyvinyl phenol resin is preferred.
The novolak resin may be one prepared by polycondensing at least one member selected from aromatic hydrocarbons such as phenol, m-cresol, o-cresol, p-cresol, 2,5-xylenol, 3,5-xylenol, resorcinol, pyrogallol, bisphenol, bisphenol-A, trisphenol, o-ethyphenol, m-ethylphenyl, p-ethylphenol, propylphenol, n-butylphenol, t-butylphenol, 1-naphthol and 2-naphthol, with at least one aldehyde or ketone selected from aldehydes such as formaldehyde, acetoaldehyde, propionaldehyde, benzaldehyde and furfural and ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone, in the presence of an acid catalyst.
Instead of the formaldehyde and acetaldehyde, paraformaldehyde and paraldehyde may, respectively, be used. The weight average molecular weight calculated as polystyrene, measured by gel permeation chromatography (hereinafter referred to simply as GPC), of the novolak resin (the weight average molecular weight by the GPC measurement will hereinafter be referred to as Mw) is preferably from 1,000 to 15,000, more preferably from 1,500 to 10,000.
The aromatic hydrocarbon of a novolak resin may, for example, be preferably a novolak resin obtained by polycondensing at least one phenol selected from phenol, o-cresol, m-cresol, p-cresol, 2,5-xylenol, 3,5-xylenol and resorcinol, with at least one member selected from aldehydes such as formaldehyde, acetaldehyde and propionaldehyde.
Among them, preferred is a novolak resin which is a polycondensation product of an aldehyde with a phenol comprising m-cresol/p-cresol/2,5-xylenol/3,5-xylenol/resorcinol in a mixing molar ratio of 40 to 100/0 to 50/0 to 20/0 to 20/0 to 20, or with a phenol comprising phenol/m-cresol/p-cresol in a mixing molar ratio of 1 to 100/0 to 70/0 to 60. Among aldehydes, formaldehyde is particularly preferred. Further, as described hereinafter, the photosensitive composition of the present invention may further contain a solubility-suppressing agent. In such a case, preferred is a novolak resin which is a polycondensation product of an aldehyde with a phenol comprising m-cresol/p-cresol/2,5-xylenol/3,5-xylenol/resorcinol in a mixing molar ratio of 70 to 100/0 to 30/0 to 20/0 to 20, or with a phenol comprising phenol/m-cresol/p-cresol in a mixing molar ratio of 10 to 100/0 to 60/0 to 40.
The polyvinyl phenol resin may be a polymer of one or more hydroxystyrenes such as o-hydroxystyrene, m-hydroxystyrene, p-hydroxystyrene, 2-(o-hydroxyphenyl)propylene, 2-(m-hydroxyphenyl)propylene and 2-(p-hydroxyphenyl)propylene. Such a hydroxystyrene may have a substituent such as a halogen such as chlorine, bromine, iodine or fluorine, or a C1-4 alkyl group, on its aromatic ring. Accordingly, the polyvinyl phenol may be a polyvinyl phenol having a halogen or a C1-4 alkyl substituent on its aromatic ring.
The polyvinyl phenol resin is usually prepared by polymerizing one or more hydroxystyrenes which may have substituents in the presence of a radical polymerization initiator or a cationic polymerization initiator. Such a polyvinyl phenol resin may be the one subjected to partial hydrogenation. Or, it may be a resin having a part of OH groups of a polyvinyl phenol protected by e.g. t-butoxycarbonyl groups, pyranyl group, or furanyl groups. Mw of the polyvinyl phenol resin is preferably from 1,000 to 10,0000, more preferably from 1,500 to 50,000.
More preferably, the polyvinyl phenol resin is a polyvinyl phenol which may have a C1-4 alkyl substituent on its aromatic ring, particularly preferably an unsubstituted polyvinyl phenol.
If Mw of the above novolak resin or polyvinyl phenol resin is smaller than the above range, no adequate coating film tends to be obtained, and if it exceeds the above range, the solubility of the non-exposed portion in an alkali developer tends to be small, whereby a pattern tends to be hardly obtainable.
Among the above described resins, a novolak resin is particularly preferred.
The proportion of such a resin in the positive photosensitive composition comprising the above-described components (a) and (b) to be used in the present invention, is preferably from 70 to 99.9 wt %, more preferably from 80 to 99 wt %.
The photosensitive composition of the present invention may further contain, as its component, a solubility-suppressing agent (dissolution inhibitor) (c) capable of lowering the dissolution rate, in the alkali developer, of a blend comprising a light-absorbing dye (a) and the above-mentioned alkali-soluble resin (b) (such a solubility-suppressing agent (c) will hereinafter be referred to simply as a solubility-suppressing agent).
When such a solubility-suppressing agent is incorporated in the photosensitive composition of the present invention, the photosensitive composition may sometimes exhibits an excellent positive photosensitive property. The action of the solubility-suppressing agent in the composition is not necessarily clear. However, it is believed at least that the photosensitive material made of this composition not only exhibits a solubility-suppressing characteristic at a non-exposed portion against the developer by the addition of the solubility-suppressing agent, while showing no such an effect at an exposed portion, but also often exhibits a dissolution-accelerating effect i.e. an effect of increasing the contrast between the exposed portion and the non-exposed portion, whereby an excellent positive image can be formed. However, the composition of the present invention is one, of which the solubility in an alkali developer is changed by a change other than a chemical change. Accordingly, the solubility-suppressing agent should also be a compound which undergoes no chemical change by exposure. In other words, it is a compound not susceptible to a photochemical sensitizing effect by the photo-thermal conversion material.
The photosensitive composition of the present invention contains an alkali-soluble resin (b) and a light-absorbing dye (a) as essential components. Accordingly, the solubility-suppressing agent (c) is one showing an effect of suppressing the dissolution of a blend of components (a) and (b), as mentioned above. However, it is believed that such an agent serves substantially to suppress dissolution of the alkali-soluble resin (b).
The solubility-suppressing agent must be at least a compound which is capable of suppressing, by its addition, the dissolving rate, in the alkali developer, of the blend comprising the above components (a) and (b) to a level of at most 80%, and it is preferably a compound capable of suppressing the dissolution rate to a level of at most 50%, more preferably at most 30%.
As a simple method for measuring the solubility-suppressing effect, for example, a blend of predetermined amounts of the above components (a) and (b) is firstly coated on a support, and the coated support is immersed in the alkali developer, whereby the interrelation between the immersion time and the reduction in the film thickness is obtained. Then, a predetermined amount of a sample of the solubility-suppressing agent is incorporated to the above blend, then the blend is coated in the same film thickness as above, and the relation between the immersion time and the reduction in the film thickness is obtained in the same manner. From these measured values, a ratio of the dissolution rates of the two can be obtained. Thus, the effect of lowering the dissolution rate of the sample of the solubility-suppressing agent used can be measured as such a relative rate. Specific examples wherein various suppressing agents are incorporated in an amount corresponding to 20 wt % of the novolak resin, are described in Examples given hereinafter.
It has been found that a wide range of compounds can be used as effective solubility-suppressing agents for the present invention. However, such a solubility-suppressing agent is required to remain in the photosensitive layer under a stabilized condition, and it is accordingly preferably solid at room temperature under atmospheric pressure or a liquid having a boiling point of at least 180xc2x0 C. under atmospheric pressure. Such effective compounds may, for example, be sulfonic acid esters, phosphoric acid esters, aromatic carboxylic acid esters, aromatic disulfones, carboxylic anhydrides, aromatic ketones, aromatic aldehydes, aromatic amines and aromatic ethers. These compounds may be used alone or in combination as a mixture of two or more of them.
More specifically, they may, for example, be sulfonic acid esters such as ethyl benzenesulfonate, n-hexyl benzenesulfonate, phenyl benzenesulfonate, benzyl benzenesulfonate, phenylethyl benzenesulfonate, ethyl p-toluenesulfonate, t-butyl p-toluenesulfonate, n-octyl p-toluenesulfonate, 2-ethylhexyl p-toluenesulfonate, phenyl p-toluenesulfonate, phenylethyl p-toluenesulfonate, ethyl 1-naphthalenesulfonate, phenyl 2-naphthalenesulfonate, benzyl 1-naphthalenesulfonate, phenylethyl 1-naphthalenesulfonate, and bisphenyl A dimethyl sulfonate; phosphoric acid esters such as trimethyl phosphate, triethyl phosphate, tri(2-ethylhexyl)phosphate, triphenyl phosphate, tritolyl phosphate, tricresyl phosphate, and tri-(1-naphthyl)phosphate; aromatic carboxylic acid esters such as methyl benzoate, n-heptyl benzoate, phenyl benzoate, 1-naphthyl benzoate, n-octyl 1-pyridine carboxylate, and tris(n-butoxycarbonyl)-s-triazine; carboxylic anhydrides such as mono-, di- or tri-chloroacetic anhydride, phenyl succinic anhydride, maleic anhydride, phthalic anhydride, and benzoic anhydride; aromatic ketones such as benzophenone, acetophenone, benzil and 4,4xe2x80x2-dimethylaminobenzophenone; aldehydes such as p-dimethylaminobenzaldehyde, p-methoxybenzaldehyde, p-chlorobenzaldehyde, and 1-naphthoaldehyde; aromatic amines such as triphenylamine, diphenylamine, tritolylamine, and diphenylnaphthylamine; and aromatic ethers such as ethylene glycol diphenyl ether, 2-methoxynaphthalene, diphenyl ether, and 4,4xe2x80x2-diethoxybisphenol A. These compounds may be substituted by a substituent of the type not to impair the effects of the present invention, such as an alkyl group, an alkoxy group, a halogen atom or a phenyl group. Further, such a compound may have a structure in which it is combined into a polymer or a resin. For example, it may, for example, be a sulfonic acid ester supported by an ester bond on a hydroxyl group of a novolak resin or a polyvinyl phenol. Such a structure may sometimes brings about an excellent suppressing effect.
Such a solubility-suppressing agent may contain, in its structure, a compound of the type having photosensitivity to ultraviolet light, such as an o-quinone diazide group-containing compound such as an o-quinone diazide sulfonic acid ester, or an aromatic disulfone such as diphenyldisulfone, whereby an excellent image can be obtained. However, in such a case, it is usually required to carry out the operation under yellow light. Accordingly, a more preferred specific embodiment of the present invention is an embodiment wherein a solubility-suppressing agent having substantially no photosensitivity to ultraviolet light. As shown in Examples of this specification, it is a photosensitive material durable for an operation for a long period of time in an environment of white light, and such a photosensitive material will bring about a substantial merit from the practical viewpoint. Such a solubility-suppressing agent (c) which is used as the case requires, may be incorporated preferably in an amount of at most 50 wt %, more preferably at most 40 wt %, based on the total weight of the components (a) and (b).
In a case where an o-quinone diazide group-containing compound is used as the solubility-suppressing agent, if the photosensitive composition is irradiated with ultraviolet ray, a positive image can be obtained in the same manner as the conventional composition. However, the photosensitive composition of the present invention is advantageously characterized in forming an image by a light within a wavelength region of from 650 to 1300 nm, and it is believed that within this wavelength region, no substantial reaction for photo decomposition of the o-quinone diazide group-containing compound will take place. This is evident also from the disclosure in JP-A-60-175046 reading xe2x80x9cin contrast to quinone diazide and diazonium salt which can not be sensitized or can only slightly be sensitized, an onium salt can readily be sensitized by a wide range of compounds over the entire visible and infrared regions of an electromagnetic spectrumxe2x80x9d. However, it is known that a 1,2-diazoketone such as an o-quinone diazide group-containing compound, undergoes a decomposition reaction also by heat. Accordingly, it is likely that when a light within a wavelength region of from 650 to 1300 nm is irradiated, it may be decomposed by the heat converted by a light-absorbing dye, and as a result, an increase in the alkali solubility of the exposed portion may be brought about.
It should be understood that in the present invention, the difference in the solubility in the developer as between an exposed portion and a non-exposed portion is essentially accomplished by a combination of the light-absorbing dye and the high molecular compound, of which the solubility in an alkali developer varies depending upon the light absorption of the dye.
An o-quinone diazide group-containing compound has absorption in an ultraviolet to visible region. Accordingly, if such an o-quinone diazide group-containing compound is used as the solubility-suppressing agent, it is usually required to carry out the operation under yellow light. However, such a compound may often bring about a desirable burning property. Such an o-quinone diazide group-containing compound may, for example, be preferably an ester compound of o-quinone diazide sulfonic acid with various aromatic polyhydroxy compounds or with a polycondensed resin of a phenol and an aldehyde or ketone.
The phenol may, for example, be a monohydric phenol such as phenol, o-cresol, m-cresol, p-cresol, 3,5-xylenol, carbacrol or thimol, a dihydric phenol such as catechol, resorcinol or hydroquinone, or a trihydric phenol such as pyrogallol or fluoroglucine. The aldehyde may, for, example, be formaldehyde, benzaldehyde, acetaldehyde, croton aldehyde or furfural. Among them, preferred are formaldehyde and benzaldehyde. The ketone may, for example, be acetone or methyl ethyl ketone.
Specific examples of the polycondensed resin include a phenol/formaldehyde resin, a m-cresol/formaldehyde resin, a m- and p-mixed cresol/formaldehyde resin, a resorcinol/benzaldehyde resin, and a pyrogallol/acetone resin. The molecular weight (Mw) of such a polycondensed resin is preferably from 1,000 to 10,000, more preferably from 1,500 to 5,000.
The condensation ratio of o-quinone diazide sulfonic acid to the OH group of a phenol group of the above o-quinone diazide compound (the reaction ratio per one OH group) is preferably from 5 to 80%, more preferably from 10 to 45%.
Among the o-quinone diazide compounds, particularly preferred is an o-quinone diazide compound obtained by reacting 1,2-naphthoquinone diazide sulfonyl chloride with a pyrogallol acetone resin.
The photosensitive composition of the present invention is prepared usually by dissolving the above described various components in a suitable solvent. The solvent is not particularly limited so long as it is a solvent which presents an excellent coating film property and provides sufficient solubility for the components used. It may, for example, be a cellosolve solvent such as methylcellosolve, ethylcellosolve, methylcellosolve acetate or ethylcellosolve acetate, a propylene glycol solvent such as propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monobutyl ether, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, propylene glycol monobutyl ether acetate or dipropylene glycol dimethyl ether, an ester solvent such as butyl acetate, amyl acetate, ethyl butyrate, butyl butylate, diethyl oxalate, ethyl pyruvate, methyl-2-hydroxy butyrate, ethyl acetate, methyl lactate, ethyl lactate or methyl 3-methoxypropionate, an alcohol solvent such as heptanol, hexanol, diacetone alcohol or furfuryl alcohol, a ketone solvent such as cyclohexanone or methyl amyl ketone, a highly polar solvent such as dimethyl formamide, dimethyl acetamide or n-methyl pyrrolidone, or a solvent mixture thereof, or the one having an aromatic hydrocarbon added thereto. The proportion of the solvent is usually within a range of from 1 to 20 times in a weight ratio to the total amount of the photosensitive material.
The photosensitive composition of the present invention may contain various additives, such as a dye, a pigment, a coating property-improving agent, a development-improving agent, an adhesion-improving agent, a sensitivity-improving agent, an oleophilic agent, etc. within a range not to impair the performance of the composition.
As a method for coating the photosensitive composition on the surface of a support, to be used in the present invention, a conventional method such as rotational coating, wire bar coating, dip coating, air knife coating, roll coating, blade coating or curtain coating may, for example, be employed. The coated amount varies depending upon the particular use, but is usually preferably from 0.1 to 10.0 g/m2 (as the solid content). The temperature for drying is, for example, from 20 to 150xc2x0 C., preferably from 30 to 120xc2x0 C. The support on which a photosensitive layer made of the photosensitive composition of the present invention will be formed, may, for example, be a metal plate of e.g. aluminum, zinc, steel or copper, a metal plate having chromium, zinc, copper, nickel, aluminum, iron or the like plated or vapor-deposited thereon, a paper sheet, a plastic film, a glass sheet, a resin-coated paper sheet, a paper sheet having a metal foil such as an aluminum foil bonded thereto, or a plastic film having hydrophilic treatment applied thereto. Among them, preferred is an aluminum plate. As the support for a photosensitive lithographic printing plate of the present invention, it is particularly preferred to employ an aluminum plate having grain treatment applied by brush polishing or electrolytic etching in a hydrochloric acid or nitric acid solution, having anodizing treatment applied in a sulfuric acid solvent and, if necessary, having surface treatment such as pore sealing treatment applied.
The light source for image exposure of the photosensitive lithographic printing plate of the present invention is preferably a light source for generating a near infrared laser beam of from 650 to 1,300 nm. For example, a YAG laser, a semiconductor laser or LED may be mentioned. Particularly preferred is a semiconductor laser or a YAG laser which is small in size and has a long useful life. With such a laser light source, scanning exposure is usually carried out, and then development is carried out with a developer to obtain a lithographic printing plate having a developed image.
The laser light source is used to scan the surface of a photosensitive material in the form of a high intensity light ray (beam) focused by a lens, and the sensitivity characteristic (mJ/cm2) of the positive lithographic printing plate of the present invention responding thereto may sometimes depend on the light intensity (mJ/sxc2x7cm2) of the laser beam received at the surface of the photosensitive material. Here, the light intensity (mJ/sxc2x7cm2) of the laser beam can be determined by measuring the energy per unit time (mJ/s) of the laser beam on the printing plate by a light power meter, measuring also the beam diameter (the irradiation area: cm2) on the surface of the photosensitive material, and dividing the energy per unit time by the irradiation area. The irradiation area of the laser beam is usually defined by the area of the portion exceeding l/e2 intensity of the laser peak intensity, but it may simply be measured by sensitizing the photosensitive material showing reciprocity law.
The light intensity of the light source to be used in the present invention is preferably at least 2.0xc3x97106 mJ/sxc2x7cm2, more preferably at least 1.0xc3x97107 mJ/sxc2x7cm2. If the light intensity is within the above range, the sensitivity characteristic of the positive lithographic printing plate of the present invention can be improved, and the scanning exposure time can be shortened, such being practically very advantageous.
As the developer to be used for developing the photosensitive lithographic printing plate of the present invention, an alkali developer composed mainly of an aqueous alkali solution is preferred.
As the alkali developer, an aqueous solution of an alkali metal salt such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium metasilicate, potassium metasilicate, sodium secondary phosphate or sodium tertiary phosphate, may, for example, be mentioned. The concentration of the alkali metal salt is preferably from 0.1 to 20 wt %. Further, an anionic surfactant, an amphoteric surfactant or an organic solvent such as an alcohol, may be added to the developer, as the case requires.
Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted to such specific Examples.
The esterification ratio in Examples was obtained from the charged ratio.
Preparation of a Lithographic Printing Plate
Preparation of an Aluminum Plate (I)
An aluminum plate (material: 1050, hardness: E16) having a thickness of 0.24 mm was subjected to degreasing treatment at 60xc2x0 C. for one minute in a 5 wt % sodium hydroxide aqueous solution and then to electrolytic etching treatment in an aqueous hydrochloric acid solution having a concentration of 0.5 mol/l at a temperature of 25xc2x0 C. at a current density of 60 A/dm2 for a treating time of 30 seconds. Then, it was subjected to desmut treatment in a 5 wt % sodium hydroxide aqueous solution at 60xc2x0 C. for 10 seconds and then to anodizing treatment in a 20 wt % sulfuric acid solution at a temperature of 20xc2x0 C. at a current density of 3 A/dm2 for a treating time of one minute. Further, it was subjected to a hydrothermal pore sealing treatment with hot water of 80xc2x0 C. for 20 seconds to obtain an aluminum plate (I) as a support for a lithographic printing plate.